Wind turbines are designed to convert wind into electricity, by turning a generator positioned in a wind turbine housing, also known as a nacelle. The rotation of the generator is achieved by wind turbine blades, normally three, that rotate by the wind. In order to be able to optimize the output power of the wind turbine, the blades may be rotated around their longitudinal axis. In this way, the blades can be used to control the amount of wind power transferred from the wind to the generator.
In conventional wind turbines, a slewing bearing is often used for rotational support of each turbine blade. An outer ring of the bearing is mounted to the hub with many bolts, typically 50 to 80 bolts, which are pre-tensioned according to specification. When the blade is assembled, it is mounted to an inner ring of the bearing with a matching number of bolts that are also pre-tensioned according to specification. Such a bearing may be a roller bearing having a diameter in the same order as the outer diameter of the blade root. In pitch control, the angular adjustment is a back and forth oscillating motion, through angles much smaller than 360 degrees. A slewing bearing is not optimally adapted for oscillations, and is prone to wear and fatigue when used for active pitch control.
Also, the pitch bearing is exposed to several different kinds of load, both static and dynamic. The loads include radial and axial forces and a bending moment due to the length and mass of the blade. A further example of a pitch bearing is known from WO 2007/112748, which discloses a bearing with a first and second axial row of rolling elements and at least one radial row of bearing elements, positioned outside of an area defined by the first and second axial row. Such a bearing is better adapted for supporting a combination of axial and radial loads, but is not particularly suited for oscillating pitch motions and will therefore also be prone to fatigue and wear when employed for active pitch control.
Consequently, there is room for improvement.